A wall on the left too

Challenge 3 of 16

This ball sets off to the left, and there is nothing at that end to catch it. A second if with its own comment does the same job on the left as the first one does on the right. It is 80 across, so its middle stops at 40.

Set the left wall.

Your code JavaScript

Set the left wall.

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Target

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Penalties: none this run

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Lesson Bouncing

The whole trick is a minus sign.

speed = -speed; takes whatever the speed is and makes it the opposite: 12 becomes -12, and -12 becomes 12. Nothing moves the ball backwards. The ball keeps doing exactly what it always did, adding its speed, and the speed is what changed.

What if does.

if (x > 360) { speed = -speed; } means: only do the thing in the curly brackets when the test in the round brackets is true. Every if in this topic is written for you, with a comment above it saying what it is watching for, so you never have to write one yourself. You choose the numbers.

One wall is never enough.

A ball that only checks the right-hand side turns around once and then leaves on the left, forever. Each edge you want it to come back from needs its own if — which is why the later challenges have four of them, one per side.

Use the arrows.

The turn happens in a single frame, and at normal speed you cannot see it. Step one frame at a time with the arrows either side of the playbar and watch the readout: the position stops climbing and starts falling, and nothing else on the line changed.